the array is not accessed outside of base64dec() so it makes sense to
limit it's scope to the related function. the static-storage duration of
the array is kept intact.
this also removes unnecessary explicit zeroing from the start and end of
the array. anything that wasn't explicitly zero-ed will now be
implicitly zero-ed instead.
the validity of the new array can be easily confirmed via running this
trivial loop:
for (int i = 0; i < 255; ++i)
assert(base64_digits[i] == base64_digits_old[i]);
lastly, as pointed out by Roberto, the array needs to have 256 elements
in order to able access it as any unsigned char as an index; the
previous array had 255.
however, this array will only be accessed at indexes which are
isprint() || '=' (see `base64dec_getc()`), so reducing the size of the
array to the highest printable ascii char (127 AFAIK) + 1 might also be
a valid strategy.